Chromium-6 (Hexavalent Chromium)

Not just Hinkley — California has thousands of community water systems with detected hexavalent chromium and only just adopted an enforceable limit in 2024

EPA MCL

Federal: 100 μg/L total chromium (almost never the limiting factor for Cr(VI)). California state MCL: 10 μg/L Cr(VI), adopted 2024 — first US state-specific limit.

Health concern

Stomach cancer (oral); lung cancer (inhaled); DNA damage at chronic low-level exposure

Testing method

EPA Method 218.6 or 218.7 (Cr(VI) specific); $50-100 per sample; not on standard well panels

Hexavalent chromium — Cr(VI) — is the carcinogenic form of chromium that the Erin Brockovich case made famous in the 1990s. Most people who heard the story took away the wrong lesson: that this was an industrial pollutant from the PG&E Hinkley site, an unusual situation. The reality is that Cr(VI) is widespread in US groundwater, much of it from natural sources, and the federal regulatory system bundles it with the chemically distinct (and biologically essential) trivalent chromium Cr(III) at a level that effectively gives Cr(VI) a free pass.

California adopted a state-specific Cr(VI) MCL of 10 μg/L in 2024 — the first state in the US to do so. Several thousand California community water systems have measurable Cr(VI), and a substantial number exceed 10 μg/L. Outside California, Cr(VI) testing is rare, and where it's been done — parts of North Carolina, Nevada, Arizona — the same pattern emerges.

Cr(VI) vs Cr(III): two different metals

Chromium exists in groundwater in two main oxidation states:

• Cr(III), trivalent chromium — relatively insoluble, weakly bioavailable, biologically essential at trace levels (cofactor for insulin signaling). Not a meaningful health concern at any concentration found in drinking water.
• Cr(VI), hexavalent chromium — soluble, mobile, carcinogenic. Crosses cell membranes readily; once inside cells, generates DNA-damaging reactive species during reduction back to Cr(III). The IARC classifies inhaled Cr(VI) as a Group 1 carcinogen and ingested Cr(VI) as Group 2A (probably carcinogenic in humans), with growing evidence supporting full Group 1 classification.

The federal EPA standard is 100 μg/L for total chromium — Cr(III) plus Cr(VI). Most groundwater chromium in regions with naturally occurring contamination is dominantly Cr(VI), so the total-chromium MCL provides almost no protection. A well at 50 μg/L total chromium that is 90% Cr(VI) is at 45 μg/L Cr(VI) — over four times California's enforceable standard but federally compliant.

Where it comes from

Two distinct source categories:

Natural sources:

• Ultramafic and serpentine rocks — chromium-rich igneous rocks present in the California Coast Range (a major source for the Central Valley aquifer), the Klamath Mountains of southern Oregon, and scattered locations in the Appalachians.
• Volcanic terrain — chromium dissolves from weathered basalts and rhyolites under oxidizing groundwater conditions. Parts of Nevada, Arizona, Utah, and the Pacific Northwest.
• Specific aquifer chemistries — Cr(III) naturally present in sediments oxidizes to Cr(VI) in well-oxygenated groundwater with elevated pH and manganese-oxide content. The Central Valley's aerated, alkaline groundwater is particularly favorable for this.

Industrial-legacy sources:

• Chrome plating — historically widespread; aerospace and metal-finishing operations with poor containment created groundwater plumes that persist for decades.
• Cooling towers — Cr(VI) was used as a corrosion inhibitor in industrial cooling water until the 1990s. PG&E's Hinkley compressor station (the Brockovich site) was a cooling-tower-discharge case.
• Leather tanning — Cr(III)-based now, but historically some Cr(VI); legacy soils can re-mobilize.
• Pulp and dye manufacturing.

Hot zones

• California Central Valley — naturally elevated Cr(VI) across substantial areas; the Hinkley case was one of dozens of documented exceedances. The state MCL adoption brings hundreds of public systems and thousands of private wells into a treatment requirement.
• San Fernando Valley and Los Angeles Basin — industrial-legacy plumes from aerospace and chrome-plating sites overlay the natural background.
• North Carolina Piedmont — naturally elevated Cr(VI) in groundwater from serpentine outcrops; local concern around coal-ash disposal sites that may also contribute industrial Cr(VI).
• Nevada (Davis Dam area, parts of Las Vegas Valley) — natural plus industrial.
• Other ultramafic terrain in OR, WA, parts of the Appalachians (rare but present).

Health effects

The carcinogenicity is well-established:

• Stomach cancer — the strongest evidence for ingested Cr(VI) comes from a 2008 NTP rodent study (clear elevated incidence) and a 2011 Greek epidemiological study (Sun et al.) of communities exposed to natural Cr(VI) in drinking water.
• Lung cancer — overwhelmingly documented in occupational inhalation cohorts (chrome platers, welders); relevance to drinking-water exposure is via aerosolized water (showers).
• DNA damage and oxidative stress — measurable at chronic exposures well below 10 μg/L; the molecular mechanism is well-understood.

The dose-response curve is uncertain at low chronic exposures, which is why California's 10 μg/L standard was politically contested for over a decade before adoption. The OEHHA's public-health goal is much lower — 0.02 μg/L — based on cancer risk modeling. The 10 μg/L MCL is a treatment-feasibility compromise, not a health-based standard.

Testing

Cr(VI) is not on standard well-water panels. You have to ask for it specifically:

• Method: EPA Method 218.6 or 218.7. Both are Cr(VI)-specific; total-chromium tests do not distinguish.
• Cost: $50-100 per sample by certified lab. Roughly double the cost of a basic metals test.
• When: at well purchase if you're in a hot zone (California Central Valley, NC Piedmont serpentine areas, ultramafic regions), and after any nearby industrial activity that might mobilize chromium.
• Sample handling matters: Cr(VI) can be reduced to Cr(III) (and thus disappear from the test) if the sample sits unpreserved. Use the lab's specific Cr(VI) sample protocol, not a generic metals bottle.

Treatment

Treatment for Cr(VI) is more involved than for most metals because the chemistry of removal depends on first reducing Cr(VI) to Cr(III), then capturing the precipitated Cr(III):

• Reverse osmosis — removes 90%+ of Cr(VI) directly. Best at point-of-use. Whole-house RO expensive.
• Anion exchange (with strong-base resin) — works for Cr(VI) (which exists as chromate or dichromate anions) but eventually saturates and needs disposal of Cr-laden resin as hazardous waste.
• Reduction-coagulation-filtration (RCF) — the standard utility-scale treatment: dose with ferrous iron or sulfite to reduce Cr(VI) to Cr(III), coagulate, filter. Works at residential scale but rarely deployed there.
• Stannous chloride or other reducing agents — sometimes used as part of multi-stage treatment.

Standard activated carbon does not remove Cr(VI). Standard water softeners do not remove Cr(VI). If you've installed a softener and you're in a Cr(VI) hot zone, your softener is doing nothing for the chromium.

Aquifers where this is a concern